Sometimes a 75 million year long story begins with one tiny thing. A tooth just barely sticking out of a rock, almost nothing, the kind of detail that in a site that is difficult to excavate can already be enough to make a paleontologist happy. Then someone looks better into the crack and realizes that underneath that fragment there is something else: bones, skull, jaws, parts of the skeleton. From that small point Cimolodon desosai re-emerged, a prehistoric mammal about the size of a golden hamster, which lived when dinosaurs were still the bulky presence on the planet. The fossil was found in Baja California, the long Mexican peninsula overlooking the Pacific, separated from US California and much more interesting, this time, for what it preserved in the stone than for the ocean postcards.
A hamster in the Cretaceous
Cimolodon desosai belonged to the multituberculates, a now extinct group of mammals that appeared in the Jurassic and remained on Earth for over 100 million years. The name may seem like bad question material, but the idea is simple: they were small mammals with distinctive teeth, often described as rodent-like in general appearance and lifestyle, although the relationship to modern rodents should be handled with caution. They lived in the folds of a world occupied by dinosaurs, moving low, among branches, soil, fruits, insects and quick shelters. In a scene like this, size mattered less than ability to get by.
The new prehistoric mammal was identified by a team led by researchers from the University of Washington. The species was described in a study published on April 22, 2026 in Journal of Vertebrate Paleontologystarting from a specimen dating back to approximately 75 million years ago. We are in the Upper Cretaceous, the last great season of the dinosaur era, several million years before the impact that 66 million years ago would have wiped out approximately 75% of life on Earth. The value of the discovery lies right there: Cimolodon desosai helps to closely observe a line of small mammals linked to those forms that managed to survive the catastrophe.
According to Gregory Wilson Mantilla, professor of biology and curator of vertebrate paleontology at the Burke Museum, the Cimolodon genus was quite widespread in the Late Cretaceous. Its fossils have been found across a very large area of western North America, from western Canada to Mexico. The new species, however, adds a particular piece: it was ancestral compared to the species that survived the mass extinction. Its small size and omnivorous diet, probably made up of fruits and insects, are considered two favorable characteristics in a world that, at a certain point, changed everything together.
Bones, teeth and a CT scan
The paleontology of such ancient mammals often works on very little. A tooth, a fragment of jaw, a trace sufficient to say something and insufficient to tell everything. In this case, however, the fossil yielded much more: teeth, skull, mandibles, a femur, an ulna and other parts of the skeleton. For an animal so small and so ancient, that’s rare luck. The bones allow us to go beyond simple identification and better imagine the body, the movements, the environment in which it lived. Cimolodon desosai, according to the researchers’ reconstruction, could move both on the ground and in the trees, with that practical flexibility that in nature is worth more than many armours.
To study it, the team used digital imaging and micro-CT, a very high resolution computed tomography scan applied to the fossil. A kind of CT scan to look inside the stone and into the smallest structures without destroying them. Then came the comparison with the teeth of other species of the Cimolodon genus, a decisive step in establishing that it was truly a new species. In animals so distant in time, the dentition is often the most reliable identity card: it resists, preserves details, allows comparisons. When the skeleton is there and the teeth are missing, giving a precise name becomes much more complicated.
The fossil was discovered in 2009 by Michael de Sosa VI, field assistant of the research group. The species name, desosai, is a tribute to him. De Sosa died while the specimen was still being studied, and the fact that that small mammal bears his name adds a very concrete human nuance to the discovery: behind every bone pulled out of the rock there is always someone who bent down, looked better, had patience. Science sometimes advances like this, with a tiny tooth and a person attentive enough to notice.
The strength of the little ones
The charm of Cimolodon desosai also lies in reducing our idea of survival. When we think of the end of the dinosaurs, we tend to only imagine the crash, the darkened skies, the collapse of ecosystems, the disappearing giants. In that same story, however, there were small, mobile, less specialized animals, capable of eating different things and using multiple environments. The prehistoric mammal found in Mexico belongs to that lateral world that rarely occupies the scene, yet holds an important part of the sequel. After extinction, mammals would have had room to diversify into increasingly varied forms. The Mexican fossil helps us understand what some of those distant relatives were like before the planet changed tune.
The authors of the study include, in addition to Wilson Mantilla, Isiah Newbins of the University of Washington, David Fastovsky of the University of Rhode Island, Yue Zhang, Meng Chen, Marisol Montellano-Ballesteros and Dalia García Alcántara of the Universidad Nacional Autónoma de México. The work was funded by UC MEXUS-CONACYT, the Dirección General de Asuntos del Personal Académico PAPIIT IN111209-2, the College of Arts and Sciences, and the Department of Biology at the University of Washington, as well as the American Philosophical Society. They are details from the laboratory, from research bureaucracy, from notes that often slip away. In this case they serve to remember how many hands and how many steps are needed to give a name to an animal that has remained frozen in the rock for millions of years.
Cimolodon desosai was small, omnivorous and probably agile among the ground and trees. Precisely such unspectacular characteristics may have helped some mammals through one of the harshest crises in the history of life.
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